1. Field of the Invention
The present invention relates to a highly flexible stent placed in a luminal structure of a living body in order to expand lumen.
2. Related Art
In a biological organ having a luminal structure such as blood vessels, the trachea and the intestines, when stenosis occurs therein, a cylinder-shaped stent with mesh pattern is used in order to secure patency at a site of pathology by expanding an inner cavity at a narrowed part. These biological organs often have bent or tapered structures locally (i.e. a tubular structure of which sectional diameters of the inner cavity differ locally in an axial direction). Therefore, a stent having higher conformability has been desired which can flexibly adapt to such a complex vessel structure. Furthermore, in recent years, stents have come to also be employed for the treatment of cerebral blood vessels. Among tubular organs in a living body, the cerebral vessel system has a more complex structure. The cerebral vessel system has many bent sites and sites having tapered structures. Therefore, stents with particularly higher conformability have been required therein.
For the purpose of realizing a stent with higher conformability, the two kinds of mechanical flexibilities of a longitudinal axis direction (in a central axis direction) and a radial direction (a direction perpendicular to the longitudinal direction) of the stent are said to be important. Thereamong, the flexibility in a longitudinal axis direction refers to stiffness with respect to bending along a longitudinal axis direction or the ease of bending. The flexibility in a radial direction refers to stiffness with respect to expansion and contraction along a direction perpendicular to a longitudinal axis direction or the ease of expansion and contraction. The mechanical flexibility in a longitudinal axis direction is a property that is necessary for a stent to be flexibly bent along a longitudinal axis direction to allow adapting to a bent site of a tubular organ in a body. The mechanical flexibility in a radial direction is a property that is necessary for making the radius of a stent flexibly differ following the shape of an outer wall of a luminal structure of a tubular organ in a body so that the stent is in tight contact with the outer wall of the luminal structure. More specifically, regarding the latter, the flexibility in the radial direction, with consideration of not only a stent having lower stiffness, but also the stent being placed in an organ in a body having a tapered structure, it is necessary for a stent to have a property whereby the expansive force of the stent does not change greatly depending on local changes in sectional diameters of the inner cavity at a site having a tapered structure.
In addition, for the stent treatment of a tubular organ of a narrow and complicated structure such as cerebral blood vessels, a stent has been desired which has a superior diameter reduction property, deliverability, and expansion so that the stent can be delivered through a network of a fine tubular organ and expanded properly at a site of pathology. Normally, an endovascular stent is reduced radially (crimped) from a state of being expanded and delivered in a catheter of small diameter that is inserted to a site of pathology in a state of being radially reduced. Then, the endovascular stent is extruded by an extruder such as a pusher from the catheter and expanded at the site of pathology. While the ratio of diameter reduction for a stent employed for conventional stent treatment of carotid arteries and crural arteries is 1/6, the ratio of diameter reduction for the treatment of cerebral blood vessels must be equal, at lowest, to or less than 1/10. For example, a vessel diameter for which the stent treatment for the intracranial circulation is employed is approximately 2.5 mm to 3.5 mm. There may be a case in which a catheter of large diameter cannot be adapted for the stent treatment due to access being limited. Furthermore, as the diameter of a catheter becomes larger, the stiffness thereof becomes greater, a result of which there is a risk of the catheter causing excessive deformation or load on vessels during the delivery of the stent. If the catheter cannot accommodate a stent of small diameter, it is necessary to user a catheter of larger diameter. For this reason, in a case of employing a stent, specifically for the treatment of cerebral blood vessels, it is necessary to employ a stent which is radially reduced (crimped) and can be accommodated in a small catheter of equal to or less than 1 mm. On the other hand, in a case of a stent for cerebral blood vessels, since it is designed such that a radial force is inherently low, a stent having an outer diameter of 1 to 2 mm greater than a vascular diameter is normally employed taking consideration of the contact between the stent and the inner wall of a blood vessel weakening. Therefore, when considering application of a stent to the treatment of an intracranial blood vessel, a stent having a high diameter reduction ratio is required. Furthermore, in the stent treatment for cerebral blood vessels, it is necessary to deliver a stent that is reduced radially to a site of pathology through a lumen of a catheter placed in a tiny and tortuous blood vessel, and thus the stent is required to pass through the lumen of a tortuous catheter. Therefore, a stent has been desired which has higher deliverability, which is arrived at by securing flexibility in a state of being reduced radially. Moreover, in order to extrude and expand a stent from a catheter at a desired site of pathology by a pusher, it has been desired such that an extrusion force can be transmitted to the stent efficiently in a longitudinal axis direction.